1. Technical Field
The present invention relates to a distributed antenna system, a base station device, and an antenna selection control method, and more particularly to a distributed antenna system, a base station device, and an antenna selection control method intended for an MIMO using a distributed antenna system (DAS) switch.
2. Background Art
In order to improve a radio environment within a building having a difficulty in receiving radio waves, there has been known a distributed antenna system (DAS) in which antennas are distributed within the building. In the related-art distributed antenna system, there has been known a method in which a leakage coaxial cable is extended from a base station device, and radio waves are radiated around the laid cable. Also, there has been known a method in which analog transmission signals input or output by the base station device are distributed to plural antennas by the coaxial cable through a device that decouples and couples the analog transmission signal. In the related-art distributed antenna system, because the input/output signal of the base station device is distributed to the plural antennas, the same signal is input and output with respect to all of the antennas. In recent years, a fast wireless communication such as a long term evolution (LTE) or a worldwide interoperability for microwave access (WiMAX) is about to be prepared. Those communication systems employ a multiple input multiple output (MIMO) technology that transmits data from plural antennas, and receives data with the plural antennas from the viewpoint of an improvement in frequency usage efficiency. In this MIMO technology, because there is a need to transmit and receive different signals with respect to the plural antennas, the different signals have been also required to be transmitted or received with respect to the plural antennas even in the distributed antenna system.
JP-A-2010-068496 discloses a background art in which the MIMO technology is applied to the distributed antenna system. According to the disclosure of this publication, “A power measurement unit of a terminal measures a received power of a pilot signal transmitted from each antenna of the distributed antenna system for a long time. The terminal selects a given number of antennas high in the received power as communication antenna candidates, and notifies a radio base station device of the communication antenna candidates and the corresponding received powers. A channel estimate unit of the terminal receives information on communicatable antennas and antenna index information allocated to the communicatable antennas from the radio base station device, and estimate channels of the communicatable antennas. In order to conduct the MIMO communication, there is a need to determine a precoding matrix that computes a transmission signal of the base station device. The terminal finds a precoding matrix index (PMI) related to the communicatable antennas on the basis of the channel estimation of the terminal, and notifies the radio base station device of the precoding matrix index with the help of the antenna index information through communication, to thereby control a phase rotation amount and a power ratio of each antenna that executes data communication.”
Also, JP-A-2007-53768 discloses as follows: “uplink information from each radio access unit is estimated to acquire a distance attenuation between the radio access unit and each user. A distance attenuation estimator stores the respective distance attenuations in association with numbers of the radio access units. A resource allocator acquires resource allocation information including information on the radio access unit employed by the user on the basis of the stored distance attenuations and numbers of the radio access units. A transmission processor conducts corresponding transmission signal processing on the user data on the basis of the resource allocation information, and generates a baseband signal stream for the radio access unit employed by the user. A radio access unit selector selects a corresponding radio access unit from all of the radio access units connected to a central processing unit on the basis of the resource allocation information, and transmits each baseband signal stream processed by the transmission processor to the selected radio access unit. Then, radio access unit conducts up-convert processing on baseband signal stream output from the radio access unit selector, and transmits the baseband signal stream subjected to the up-convert processing to the corresponding radio access unit through a transmission link as downlink information.”